Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211635

1135 AM CDT Mon Apr 21 2014

.SHORT TERM: (Today through Thursday)
Issued at 400 AM CDT Mon Apr 21 2014

Two shortwave trofs are coming into phase over the Great Plains
this morning. A relatively weak low level jet is pointed at the
Mid-Mississippi Valley as a result with only around 15kts at
850mb. Even though the jet is rather weak in the grand scheme of
things, there appears to be enough isentropic lift and moisture
convergence associated with the first vortmax ejecting northeast
from the trof across southwest Missouri to produce a rather large
area of light to moderate rain. Short-range guidance including the
RAP, HRRR, and 4km NCEP WRF push this area of showers up into our
CWFA by 12Z. Multiple runs of these models have shown this trend,
and the precip across southwest Missouri has been increasing in
coverage and intensity (a bit) over the past hour or so. See no
reason to disbelieve the guidance at this time, so have bumped up
pops this morning...primarily over central and parts of southeast

The northern shortwave is dragging a cold front southeast through
Nebraska at this time and the front will enter the northwestern
portions of our CWFA between 21-00Z this evening.  Expect
clouds/precip this morning to keep temperatures from warming up as
much as yesterday.  However, most of if not the entire CWFA should
stay in the warm sector today, so have leaned toward warmer MAV
guidance.  It won`t take much insolation to push temperatures into
the mid and possibly upper 70s especially since it looks like we`ll
be starting the day in the upper 50s to low 60s across most of the
area.  Still expecting scattered to numerous showers and
thunderstorms to develop this afternoon ahead of the cold front.
While there should be more than enough instability for storms to
occur, deep layer shear should be relatively weak (around 15-20kts),
and CAPE values will be generally 1200 J/Kg or less so severe storms
are not expected.

Convection should diminish within a few hours after sunset, and the
cold front will continue moving southeast...clearing the southeast
border of our CWFA by 06Z.  Cooler and drier air will overspread the
area behind the front.  Expect temperatures on Tuesday and Wednesday
to be near normal with highs in the mid 60s to low 70s and lows
generally in 40s to low 50s.  Meanwhile, a piece of the high
amplitude trof spinning over the eastern Pacific will come onshore
Tuesday and move across the Rockies Wednesday into the Great Plains
Wednesday night.  This will result in good moisture return ahead of
the surface front Wednesday night into Thursday.  All guidance is
printing out precip ahead of the front on Thursday.  Deep layer
shear should be in excess of 40kts with the strong trof aloft and
the cold front sweeping through during the day Thursday.  Unsure yet
what the severe potential will be as instability may be limited.


.LONG TERM: (Thursday night through Sunday)
Issued at 400 AM CDT Mon Apr 21 2014

Medium range guidance continues to have difficulty coming to a
consensus for this period.  Pattern becomes highly amplified Friday
and Saturday as the bulk of that gihemous Pacific trof comes
ashore.  The main difference between the ECMWF and GFS headed into
the weekend is the strength of the amplification of the flow over
the eastern 1/2 of the CONUS.  Both models have a decent upper level
low over southern Ontario by 00Z Saturday, but the EC opens it up as
the wave heads east into Sunday morning while the GFS keeps a
stronger closed low.  The GFS allows for a much stronger surface
ridge and colder air to flow south out of Canada into the Mid and
Lower Mississippi Valley Saturday and Sunday while the EC keeps the
cold air bottled up over the Upper Midwest.  The GEM seems to
potentially favor the EC solution in the last periods of its
forecast, but with little else to support either solution, have
stuck with the NWS Central Region ensemble blend for the forecast,
which calls for below normal temperatures and a chance of precip
Saturday night into Sunday.



.AVIATION:  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1056 AM CDT Mon Apr 21 2014

At least scattered showers and thunderstorms are expected this
afternoon into the early evening hours as an upper level
disturbance moves eastward through the region, while a cold front
moves southeastward through the taf sites late this afternoon and
this evening. Looking at the latest radar trends it appears that
most of the shower activity early this afternoon should occur in
the UIN area, but showers and storms should develop in the COU
area later this afternoon and move through the St Louis metro area
particularly during the early evening hours. Although prevailing
conditions have been VFR outside of the showers so far, some of
the MVFR cigs over western MO will likely advect into the region
later this afternoon, plus there will likely be some MVFR cigs
just behind the cold front. Mainly southwesterly winds will veer
around to a northwesterly direction after fropa. The clouds
should clear out fairly quickly this evening in UIN and COU and
overnight in the St Louis metro area as a relatively strong
surface ridge builds southeastward into the region behind the cold

Specifics for KSTL: Looks like a break in the shower activity
early this afternoon, but model guidance indicates that showers
and storms will develop and move through the STL area late this
afternoon and early this evening. These showers and storms should
shift southeast of STL by 04z Tuesday after fropa with the wind
veering around to a northwesterly direction. Will have VFR
conditions early this afternoon until the showers and storms move
into the area with the lowest prevailing cloud ceilings likely
this evening, possibly briefly down to around 1000-2000 ft. The
clouds should clear out shortly after 06z Tuesday.



Saint Louis     76  52  70  45 /  60  50   0   0
Quincy          74  47  66  42 /  60  20   0   0
Columbia        73  48  69  45 /  60  30   0   0
Jefferson City  74  50  70  44 /  70  40   0   0
Salem           74  54  67  41 /  60  60   5   0
Farmington      73  53  70  43 /  70  60   5   0




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